pom2bp: extract minSdkVersion from manifest into Android.bp

When manifest merging in Soong is turned on the modules created
by pom2bp will need to have min_sdk_version specified in order
to prevent manifest_fixer from setting it to the current SDK
level.  Read the minSdkVersion attribute out of the
AndroidManifest.xml file inside each .aar file.

Bug: 110848854
Test: cd prebuilts/sdk/current/support && pom2bp -regen Android.bp
Change-Id: Ieaf37ba6eccaf32dc5d3411566335830b1a327ff