Set default target SDK version for APEX

Target SDK version is used for targeting an APEX to a specific set of
platform builds. Usually, the targeting is unrestricted (in case the
APEX can run on all platforms), or based on platform SDK version (e.g.
28 for P). However, when the platform is under development and SDK is
not finalized, the targeting should be much more fine-grained; the
APEX should be targeted to a very specific build that supports the same
set of APIs that the APEX was built against.

To support that, target sdk version is automatically set by the build
system. When the platform is released or SDK is finalized, the target sdk
version set to the SDK version number. If not, it is set to
<version_code>.<fingerprint> (e.g., Q.123456).
Note that the target sdk version set by the build system is used only
when the target sdk version is not explicitly set in
AndroidManifest.xml.

Bug: 130541924
Test: UNBUNDLED_BUILD_SDKS_FROM_SOURCE=true \
UNBUNDLED_BUILD_TARGET_SDK_WITH_API_FINGERPRINT=true \
TARGET_BUILD_APPS=com.android.tzdata m
build.ninja has --target_sdk_version Q.$$(cat out/soong/api_fingerprint.txt)

Test: aapt dump badging out/dist/com.android.tzdata.apex | grep \
targetSdkVersion shows:
targetSdkVersion:'Q.6ee443d9ad5f0cca7a43cfa97b7fc62a'

Change-Id: I086230d787f01075c28fc3f0163550300fa00212