AAPT2: Add targetConfig to AndroidManifest.xml

Splits may include more/different configurations than they
were originally specified to include. This happens, for instance,
when a particular requested density doesn't exist. A fallback density
resource is chosen for the split.

This makes programmatic configuration targetting difficult, as the APK
may report that it has resources for multiple densities, while it was
meant to target only a specific density.

This change encodes a comma separated list of the intended configurations
for the split in an attribute called 'targetConfig' on the <manifest> tag of
AndroidManifest.xml.

Test: manual
Change-Id: I3167e9341a434abc34ed2ddb8f740762b40b1ab7