"Balance" button widths in miniresolver

The previous implementation assigned too much of the extra width to
only one of the buttons, making them appear imbalanced (b/386811027).
This CL assigns the buttons equal weight in a LinearLayout, which
looks OK in all the cases I tested (by manually hacking in different
text values).

Screenshots: (ignore the missing dialog text; just from my hack)

 * screenshot/B2d6hxEUie5viJs uses short placeholders on both buttons

 * screenshot/9Xm9mNiVABFHeLE and screenshot/BSZ4ukpzYW2SUqL show the
   effect when one of the button labels is much longer than the other

 * screenshot/8z8XDQLXE4YxnqJ shows when both buttons have long labels

 * screenshot/3pDaJFdDPQ8z6mm uses the real "Cancel" and "Open" labels
   from b/386811027

(* screenshot/8Ygxny5ar6Be7cH shows that the dialog text was only
   missing incidentally, and was restored when I removed my hack.)

Bug: 386811027
Test: (see screenshots)
Flag: EXEMPT bugfix
Change-Id: Ib6818a366a7d5257c3248a6eae4d50df971c8c21