Layoutlib: support viewkey in include nodes.

When an include tag is parsed, the custom bridge
code never has access to both parsers at the same
time.
The child parser is created out of an in (representing
the layout id), and the code inflating the content of
the include layout doesn't see the parent parser either.

This changeset adds a parser stack in the BridgeContext
in order to allow access to the parent parser when setting
the viewkey. This is only used if the current parser depth
is 1 (top node), as we only want to set the include
node to the top node of the included layout.

Change-Id: I2ac3b72a0c84a269d9019f44f98cbc0b615ab959