Support AVD snapshot caching

[ychescale9]

Add force-avd-creation which when set to false will skip avd creation if avd with same name exists.

Setting force-avd-creation to false allows cached AVD to be used when starting emulator which significantly reduces emulator startup time from 1-3 minutes down to ~15 seconds.

Snapshot caching can be done by running the action with snapshot (no -no-snapshot in emulator-options) if no snapshot cache is found, before running the action again (with -no-snapshot-save to make sure the cache won't be updated after running tests) to run to actual tests:

- uses: actions/cache@v2
  id: avd-cache
  with:
    path: |
      ~/.android/avd/*
      ~/.android/adb*
    key: avd-${{ matrix.api-level }}

- name: generate snapshot for caching
  if: steps.avd-cache.outputs.cache-hit != 'true'
  uses: reactivecircus/android-emulator-runner@v2
  with:
    api-level: ${{ matrix.api-level }}
    force-avd-creation: false
    emulator-options: -no-window -gpu swiftshader_indirect -noaudio -no-boot-anim -camera-back none
    disable-animations: false
    script: echo "Generated AVD snapshot for caching."

- name: run tests
  uses: reactivecircus/android-emulator-runner@v2
  with:
    api-level: ${{ matrix.api-level }}
    force-avd-creation: false
    emulator-options: -no-snapshot-save -no-window -gpu swiftshader_indirect -noaudio -no-boot-anim -camera-back none
    disable-animations: true
    script: ./gradlew connectedDebugAndroidTest

Related to #76.