Ensure Navigation Material properly handles back for nested nav

If there is a nested NavGraph under the current bottomSheet destination
and you do a back press, instead of the bottomSheet which is the topmost
destination being popped, the nested NavHost underneath will be popped
instead. This is caused by bottomSheet not properly intercepting back presses.

This change added a BackHandler to ensure that bottomsheets correctly
respect the system back hierarchy for back events.

Fixes: #1726

navigation-material/src/androidTest/java/com/google/accompanist/navigation.material/BottomSheetNavigatorTest.kt

@@ -779,6 +779,71 @@ internal class BottomSheetNavigatorTest {
             .isEqualTo(firstSheetDestination)
     }
 
+    @Test
+    fun testBackPressWithNestedGraphBehind() {
+        lateinit var navigator: BottomSheetNavigator
+        lateinit var navController: NavHostController
+        lateinit var nestedNavController: NavHostController
+        lateinit var backDispatcher: OnBackPressedDispatcher
+        val homeDestination = "home"
+        val firstSheetDestination = "sheet1"
+        val firstNestedDestination = "nested1"
+        val secondNestedDestination = "nested2"
+
+        composeTestRule.setContent {
+            backDispatcher = LocalOnBackPressedDispatcherOwner.current?.onBackPressedDispatcher!!
+            navigator = rememberBottomSheetNavigator()
+            navController = rememberNavController(navigator)
+            ModalBottomSheetLayout(navigator) {
+                NavHost(navController, homeDestination) {
+                    composable(homeDestination) {
+                        nestedNavController = rememberNavController()
+                        NavHost(nestedNavController, "nested1") {
+                            composable(firstNestedDestination) { }
+                            composable(secondNestedDestination) { }
+                        }
+                    }
+                    bottomSheet(firstSheetDestination) {
+                        Text("SheetDestination")
+                    }
+                }
+            }
+        }
+
+        assertThat(navController.currentBackStackEntry?.destination?.route)
+            .isEqualTo(homeDestination)
+
+        composeTestRule.runOnUiThread {
+            nestedNavController.navigate(secondNestedDestination)
+        }
+        composeTestRule.waitForIdle()
+
+        assertThat(navController.currentBackStackEntry?.destination?.route)
+            .isEqualTo(homeDestination)
+        assertThat(nestedNavController.currentBackStackEntry?.destination?.route)
+            .isEqualTo(secondNestedDestination)
+
+        composeTestRule.runOnUiThread {
+            navController.navigate(firstSheetDestination)
+        }
+        composeTestRule.waitForIdle()
+
+        assertThat(navigator.sheetState.currentValue)
+            .isAnyOf(ModalBottomSheetValue.HalfExpanded, ModalBottomSheetValue.Expanded)
+
+        composeTestRule.runOnUiThread {
+            backDispatcher.onBackPressed()
+        }
+        composeTestRule.waitForIdle()
+
+        assertThat(navController.currentBackStackEntry?.destination?.route)
+            .isEqualTo(homeDestination)
+        assertThat(nestedNavController.currentBackStackEntry?.destination?.route)
+            .isEqualTo(secondNestedDestination)
+
+        assertThat(navigator.sheetState.currentValue).isEqualTo(ModalBottomSheetValue.Hidden)
+    }
+
     private fun BottomSheetNavigator.createFakeDestination() =
         BottomSheetNavigator.Destination(this) {
             Text("Fake Sheet Content")

navigation-material/src/main/java/com/google/accompanist/navigation/material/BottomSheetNavigator.kt

@@ -17,6 +17,7 @@
 package com.google.accompanist.navigation.material
 
 import android.annotation.SuppressLint
+import androidx.activity.compose.BackHandler
 import androidx.compose.animation.core.AnimationSpec
 import androidx.compose.foundation.layout.ColumnScope
 import androidx.compose.material.ExperimentalMaterialApi
@@ -212,6 +213,10 @@ class BottomSheetNavigator(
             LaunchedEffect(retainedEntry) {
                 sheetState.show()
             }
+
+            BackHandler {
+                state.popWithTransition(popUpTo = retainedEntry!!, saveState = false)
+            }
         }
 
         SheetContentHost(